Technical Field
The present disclosure relates to a photoacoustic-type gas detection device and more specifically to a sensor based on a Helmholtz resonator.
Discussion of the Art
A photoacoustic gas detection device with discrete components using a sensor based on a Helmholtz resonator is described in FR2815122, FIG. 1 of this application being reproduced as FIG. 1 of the drawings.
This device comprises a light source (laser) 1 modulated by a mechanical modulator 2 at an acoustic frequency. Modulated beam 3 is sent into a tube 40 of a resonant Helmholtz cell 4 containing a gas mixture to be analyzed. This cell comprises a second tube 41 parallel to the first one. The two tubes are connected by capillaries 43 and 44. By properly selecting the length and the diameter of the tubes and of the capillaries, a cell at a selected resonance frequency can be formed. The acoustic resonance frequency is adapted to the modulation frequency imposed by modulator 2 (or conversely). An electret microphone 10, 11 is associated with each of tubes 40, 41. The microphone outputs are sent to a differential amplifier 8. The output of this amplifier provides a display system 9 with electric signals representative of the amount of gas present. The device also comprises an electronic assembly 7 enabling to control the mechanical modulator. Thus, when the laser wavelength corresponds to an absorption stripe of a gas, the presence of this gas and its concentration can be determined.
However, such a gas detection device formed based on discrete elements remains limited to laboratory applications. Indeed:                it is difficult to find materials usable with discrete elements with transmission wavelengths greater than 2.5 μm while it would be desirable for a gas analysis to be possible at wavelengths in more remote infrared, within a range from 3 to 10 μm;        the device is generally sensitive to temperature variations and to vibrations which may disturb the alignment;        the implementation of the system, that is, the positioning of its elements and their alignment, must be performed by means of very accurate optical benches, which are very difficult to handle;        the macroscopic size of the device prevents “sensor”-type applications capable of competing with non-selective chemical sensors;        it is not possible, with such a device, to scan a wide range of wavelengths and it is very difficult to replace the laser source.        
Photoacoustic gas detection devices overcoming the disadvantages of known devices are thus needed.